Laccase is a polyphenol oxidase (EC 1.10.3.2) which catalyses the oxidation of a variety of inorganic and aromatic compounds, particularly phenols, with the concomitant reduction of molecular oxygen to water.
Laccase belongs to a family of blue copper-containing oxidases which includes ascorbate oxidase and the mammalian plasma protein ceruloplasmin. All these enzymes are multi-copper-containing proteins.
Because laccases are able to catalyze the oxidation of a variety of inorganic and aromatic compounds, laccases have been suggested in many potential industrial applications such as lignin modification, paper strengthening, dye transfer inhibition in detergents, phenol polymerization, hair colouring, and waste water treatment. A major problem with the use of laccases are their poor storage stability at temperatures above room temperature, especially at 40.degree. C.
In Example 1 of the present application we have tested the stability of various laccases at 40.degree. C., and it can be seen that after 2 weeks of storage the laccase activity is down to less than 50% of the initial value, and at low pH the laccase activity after 2 weeks is zero. For many purposes such a decrease is unacceptable, so it is the purpose of the present invention to create laccase variants with improved stability by using the information of a three-dimensional structure of a Coprinus cinereus laccase. No three-dimensional structural information has been available for a laccase before.